Shared library symbol visibility

Shared library symbol visibility

[Noah Watkins]

It looks like we may be outgrowing the use of export-symbols-regex and
friends to control symbol visibility for published shared libraries.
On Linux, ld seems to be quite content linking against hidden symbols,
but at least on OSX with Clang it seems the visibility is strictly
enforced.

For instance, librados exports only the prefix "rados_", but that
regex hides everything in the C++ interface. Unfortunately,
export-symbols-regex doesn't play nice with C++ name mangling.

Large projects that I've been looking at for examples (chromium, v8,
Java) seem to use a different approach based on the compiler flag
"-fvisibility=hidden" that hides everything by default and uses
explicit exporting.

These are the basics, and there are variants that work on Windows for
DLL's as well with more macro magic.

#define CEPH_EXPORT __attribute__((__visibility__("default")))

  class CEPH_EXPORT ObjectOperation {
    public:
      ObjectOperation();
      virtual ~ObjectOperation();
      ...

There is a sample branch up with this approach at:

   http://github.com/ceph/ceph port/visibility

More info

https://www.gnu.org/software/gnulib/manual/html_node/Exported-Symbols-of-Shared-Libraries.html
http://gcc.gnu.org/wiki/Visibility

- Noah

Re: Shared library symbol visibility

[Josh Durgin]

On 12/30/2013 09:56 AM, Noah Watkins wrote:
> It looks like we may be outgrowing the use of export-symbols-regex and
> friends to control symbol visibility for published shared libraries.
> On Linux, ld seems to be quite content linking against hidden symbols,
> but at least on OSX with Clang it seems the visibility is strictly
> enforced.
>
> For instance, librados exports only the prefix "rados_", but that
> regex hides everything in the C++ interface. Unfortunately,
> export-symbols-regex doesn't play nice with C++ name mangling.
>
> Large projects that I've been looking at for examples (chromium, v8,
> Java) seem to use a different approach based on the compiler flag
> "-fvisibility=hidden" that hides everything by default and uses
> explicit exporting.
>
> These are the basics, and there are variants that work on Windows for
> DLL's as well with more macro magic.
>
> #define CEPH_EXPORT __attribute__((__visibility__("default")))
>
>    class CEPH_EXPORT ObjectOperation {
>      public:
>        ObjectOperation();
>        virtual ~ObjectOperation();
>        ...
>
> There is a sample branch up with this approach at:
>
>     http://github.com/ceph/ceph port/visibility
>
> More info
>
> https://www.gnu.org/software/gnulib/manual/html_node/Exported-Symbols-of-Shared-Libraries.html
> http://gcc.gnu.org/wiki/Visibility

This is great! It serves as a good reminder when you're touching public
interfaces as well. It may be annoying to add it to everything
initially, but it seems quite nice once it's in place!

Josh